In space, there is a need for a system that can isolate a payload from spacecraft disturbances and vibrations such as those from launch, cryo-cooling and thermal expansion. Localized vibrations in launch vehicles have always been a problem. For example, Space Shuttle Solid Rocket Boosters generate a significant amount of vibrations. These vibrations are sensed by on-board gyros and accelerometers which are supposed to measure rigid body vehicular motion. The Space Shuttle, for example, has localized resonances on the boosters near instrumentation boxes used for flight control. These resonances were detected during vibration tests and were corrected by stiffening the compartment before the first flight. This prior method of stiffening structures, changes the frequency response and hence the resonance frequencies so that the vibrations frequencies would not interact with flight control systems which could cause flight control instability problems. One disadvantage of this approach is the necessary alteration of the flight structures, if at all practicable, which may be unique with each particular flight vehicle and for each payload.
Conventional vibration isolation systems are either passive or active systems. The passive systems use springs and dampers, such as rubber washers. The active systems use gimbals, motors, magnetic suspensions or proof-mass actuators. The active systems provide improved isolation over passive systems, but the active systems are higher in costs, heavy and bulky, and are not well suited for some space applications.
The vibration attenuation through active damping may be used to protect sensitive instruments from high vibrational loads. Active isolation systems using piezoelectric actuators (PZTs) bonded to vibrating plates have demonstrated vibration attenuation of high vibrations. The PZTs are bonded to the plate and electrically controlled to reduce plate vibrations.
PZTs are piezoelectric ceramic materials that can be distorted by the application of an electric field. The PZTs material is electrically poled during manufacture by the application of a large electric field annealed at high temperature. The application of an electric field along the polarization direction forces the PZTs to expand along the directions perpendicular to the electric field producing a local strain on the plate surface. If the electric field is opposite to the polarization direction, the PZTs will contract in the direction perpendicular to the electric field. This expansion or contraction of the PZTs bonded on the plate generate a local moment on the plate surface. During application as an actuator, the PZTs are operated below the depoling field which is approximately 750 volts/mm, for example at 150 V for a 2.0 by 5.0 cm square by 0.2 mm thick PZT bonded to, for example, a 0.5 meter by 0.6 meter by 1.0 mm thick aluminum plate.
The active PZT plate vibration system has PZTs bonded to a payload support platform plate upon which may also be secured the payload. The PZTs provide local moments on the plate. The PZTs are low in weight, volume and cost and respond to high frequency electric signals between 3 to 5 KHz. Hence, the PZTs may be bonded on a plate to remove vibrational energy of the plate so as to protect the payload. Accelerometers were located near the PZTs so that the system response between the co-located PZTs and accelerometers has a minimum-phase and is positive-real. This co-located system is easier to control than systems which are not co-located.
This co-located plate isolation system may use a variety of control methods, including for example, an H-Infinity design control method, or for another example, a rate feedback control method. The rate feedback control method is known to provide dampening over a large number of resonant frequencies, that is "modes", but without significant attenuation at any specific resonant frequency. The H-Infinity control method can be used to focus attenuation control at a few specific resonant frequencies but having a more complex control.
However, these prior plate co-located vibration isolating systems have failed to actively isolate sensitive instruments or experiments from disturbances coming from vibrating tables, floors, or vehicles. The plate can not isolate vibration originating from the struts and vehicular structures, even though localized vibrations on a plate may be reduced. PZT stacks may be adapted for directional attitude control, for example, for pointing space telescopes or antenna, and bonded PZTs have been adapted for plate vibration isolation. However, bonded PZTs used on plate vibrations isolation and PZT stacks used for directional control have limited performance and applications and have not been integrated into a single vibration isolation and directional control system. These and other disadvantages are solved or reduced using the present invention.